1. Field of the Invention
The present invention relates generally to cochlear implant prostheses and, more particularly, to applying stimulation pulses to the neural structures of a cochlea in accordance with a multirate cochlear stimulation strategy.
2. Related Art
Cochlear implant systems (also commonly referred to as cochlear devices, cochlear prostheses, and the like; collectively and generally referred to herein simply as “cochlear implant systems” or “cochlear implants”) are used to aid patients (also referenced to as recipients, implantee, wearer, user, and the like) having a hearing deficiency. More particularly, these systems include a microphone receiving ambient sounds and converting the sounds into corresponding electrical signals, signal processing means for processing the electrical signals and generating cochlear stimulating signals and an electrode assembly for applying the cochlea stimulating signals to the cochlea of an implantee. In response to such stimulating signals a perception of corresponding ambient sound is elicited in the implantee.
The inner ear of a normal hearing person includes hair cells which convert the displacement of the ear's basilar membrane in response to sound into nervous impulses. Different parts of the basilar membrane of the normal cochlea are displaced maximally by different frequencies of sound so that low frequency sounds maximally displace apical portions whereas higher frequency sounds cause displacement of more basal portions of the membrane. The nervous system is arranged so that a nervous impulse originating from a hair cell located adjacent an apical area of the membrane is perceived as a low frequency sound whereas a nervous impulse originating from a hair cell located adjacent a more basal position of the membrane is perceived as a higher frequency sound. This mapping of position to pitch is well known in the art as the tonotopic arrangement of the cochlea.
In a dysfunctional ear the hair cells may be damaged or absent so that no nervous impulses are generated. In such cases electrical stimulation impulses must be provided artificially to simulate the nervous activity of the hair cells in order to create a perception of sound.
FIG. 1A is a schematic illustration of a totally implantable cochlear implant. In such cochlear implant devices, ambient sounds are detected by a microphone 103 and a transduced signal is thereby generated, representative of the ambient sound. A processor unit 105 then processes this transduced signal according to one of many strategies, (some of which will be explained further below) and based on this processing stimulation currents are applied between the electrodes of a coupled array. For example, in “monopolar” mode stimulation, stimulation currents may be caused to flow between an electrode of the electrode array 109 and an extracochlear electrode 115. Nervous discharges elicited in the basilar membrane are conveyed to the central nervous system of the wearer by auditory nerve 113.
In the event that the stimulation current flows between an apical electrode (such as electrode 111) and extracochlear electrode 115, then a lower pitched hearing sensation will be perceived by a wearer of the prosthesis than will be the case if stimulation current flows between a basal electrode (such as electrode 107) and extracochlear electrode 115 because of the tonotopic arrangement of the cochlea 101. Further pitch information may be transmitted to the wearer corresponding to the rate at which stimulations are delivered.
In the past designers of cochlear implant stimulation strategies have striven to optimize the intelligibility of spoken words as perceived by the wearer of a cochlear implant.
One of the earliest sound processing and cochlear stimulation strategies is described in U.S. Pat. No. 4,532,930 to the present applicant. In that patent there is taught the use of a filter dedicated to extracting the voice pitch of a speech signal. The periodicity of the voice pitch is used to set the stimulation periodicity for two or three electrodes. A second, and possibly third, channel is analyzed to determine periodicity and amplitude in a selected frequency band.
The periodicity extracted from the second filter, and possibly third filter, is used to select which electrode is stimulated for the second and third channels while the periodicity of stimulation of the channel is determined by the periodicity of the output signal from the first filter.
Another stimulation arrangement is described in U.S. Pat. No. 4,207,441. In that system there are n electrodes each coupled to one of n filters. Each electrode is stimulated once per analysis period, with an intensity corresponding to the amplitude of the corresponding filter channel. The analysis period of this system is predetermined and hence is not related to the signal on the filter outputs.
More recently, in EP 0 745 363 there is described a stimulation system which takes into account the temporal behavior of the cochlea. In one described embodiment a wavelet transformation is used to extract the temporal information with a view to using this information to determine the sequence of stimulation of the electrodes. The purpose of the invention is to improve the temporal resolution in response to a rapidly changing audio spectrum.
A problem commonly experienced by users of the above or other types of conventional cochlear implants featuring the above and other conventional stimulation schemes is that while intelligibility of spoken words is often good, the user's pitch perception is less than optimal. Accordingly, it is an object of the present invention to provide an apparatus and method for use in a multi-channel cochlear implant which will improve a users perception of pitch.